The torsion bar is also the central control inside a spool valve. The spool valve (seen here with the new blue seals) is what controls the pressurized fluid and determines if it should bypass the box and return to the reservoir or flow through the box and help it push a piston back and forth. As the torsion bar twists, it opens the valve to one or the other side of the piston. The spool valve is also attached to a ball screw that screws into the piston and causes it to move up or down within the piston chamber in the steering box.